International Geology Review

Chopin, Christian; Schertl, Hans-Peter

doi: 10.1080/00206819909465168pmid: N/A

New results acquired in the coesite-bearing terrane of the Dora-Maira massif, Italian Western Alps, indicate that the UHP unit extends over less than 45 km2 and is bounded by two lower-grade continental units. The contact to the footwall unit is a late, lowangle, top-to-the-SW extensional one, which probably cuts across the original thrust contact. The protolith of the pyrope “whiteschist” of the UHP unit is shown to be the metasomatic transformation product of granitic rock. Nearend member magnesiochloritoid and magnesiostaurolite occur in pyrope megablasts. Ordering in mag-nesiostaurolite leads to cross-hatched twinning and superstructures with either a doubled a or doubled c parameter. The Si3.5 phengite-3T from the same whiteschist contains nano- to microscale quartz platelets and talc layers parallel to (001) of the host mica, interpreted as in situ decompression products of the mica. Some of the pyrope megablasts contain late, crack-related granulite-facies mineral assemblages, which reflect very low H2O activities as a result of local closed-system evolution at temperatures as low as 550 to 600°C. Thermobarometry on successive mineral generations in the dark- and light-colored UHP eclogites (probably former dikes and/or volcanic tuffs with sedimentary components) better constrains the prograde path and confirms that decompression took place at temperatures just slightly higher than during burial. The geochemistry of the main rock types in this piece of subducted continental crust shows no evidence of interaction with mantle material. Localscale fluid evolution and control by deformation seem to be a general feature. New geochronological and fission-track data imply very high uplift rates, on the order of 2.2 cm/y.

Carswell, D. A.; Zhang, R. Y.

doi: 10.1080/00206819909465169pmid: N/A

Ultrahigh-pressure eclogites are metabasic rocks that have equilibrated at pressures within the coesite P-T stability field. This paper reviews their occurrences and petrographic characteristics in exhumed ultrahigh-pressure metamorphic terrenes within certain major Phanerozoic continental plate—collision belts. Unequivocal identification of ultrahigh-pressure eclogites depends on the presence of relict coesite or of polycrystalline quartz pseudomorphs after earlier coesite. However, preservation of such distinctive petrographic evidence is at best sporadic, leading to serious problems over whether or not particular eclogite samples have experienced “peak” pressures consistent with coesite stability. Accordingly, we also review here other petrographic, mineral-assemblage, and mineral-chemistry features that may aid in the identification of ultrahigh-pressure eclogites. In addition, we discuss the quantitative evaluation of P-T conditions for formation based on various reaction equilibria, including the dominant garnet and omphacite phases. Various aspects of the mineral compositions are taken to indicate that the “peak” metamorphic pressures were most likely attained at lower than “peak” temperatures. Hence it is concluded that tight, hairpin-style P-T-time paths for the prograde and retrograde stages in the metamorphic evolution of ultra-high pressure eclogites that assume synchronous achievement of Pmax and Tmax conditions, as deduced in many recently published studies, may be invalid. A rather more open style, clockwise P-T-time path, as deduced for the metamorphic evolution of a typical ultrahigh-pressure phengite-bearing eclogite sample from the Dabie Shan in central China and characterized by a significant further temperature rise during the initial phase of relatively rapid exhumation, is considered to be more realistic.

Medaris, L. G.

doi: 10.1080/00206819909465170pmid: N/A

Garnet peridotites and pyroxenites have been reported from 11 of the 15 or so high-pressure/ultrahigh-pressure (HP/UHP) terranes in Eurasia. Most of these ultramafic rocks are Mg-Cr types, derived from depleted upper mantle, but some are more Fe-rich and originated by crystallization in ultramafic-mafic igneous complexes. The peridotites are polymetamorphic, with HP/UHP garnet-bearing assemblages being followed by a succession of retrograde assemblages related to exhumation and cooling; some also contain evidence for a pre-HP/UHP stage, such as spinel inclusions in garnet or the presence of Ti-clinohumite. Equilibration conditions have been calculated from all available analyses of garnetiferous assemblages, by application of the olivine-garnet Fe-Mg exchange thermometer and the Alin-orthopyroxene barometer, resulting in two distinct P-T regimes for garnet peridotites—one at high P/T in the coesite and diamond fields, and another at low P/T in the vicinity of the spinel-to-garnet transition. Garnet peridotites are thought to have evolved in at least four different tectonothermal settings, including: (1) emplacement of peridotites into oceanic or continental crust, followed by transport of peridotites and associated crust to UHP conditions by a subducting plate; (2) transfer of peridotites from a mantle wedge to the crust of an underlying, subducting plate; (3) origination from upwelling asthenosphere that passed through a high-temperature spinel peridotite stage, followed by cooling into the garnet peridotite field; and (4) extraction of garnet peridotites from ancient subcontinental lithosphere, perhaps by deep-seated faulting within a continental plate.

Neng, Jiang; Yongfeng, Zhu

doi: 10.1080/00206819909465171pmid: N/A

Orogenic gold vein deposits in the Xiaoqinling district are situated in a basement-cored uplift along the southern margin of the North China craton. The deposits are hosted by Late Archean to Paleoproterozoic amphibolite-facies country rocks, varying in lithology from clastic and chemical sedimentary units to felsic and mafic volcanic rocks and plutons. Absolute and relative age relationships indicate a late Mesozoic emplacement of the lodes, which is subsequent to the deformation associated with the Middle to Late Triassic Qinling orogen along the craton margin. All auriferous quartz veins are hosted in faults. The ores are generally composed of quartz veins with various amounts of pyrite, galena, chalcopyrite, sphalerite, and carbonates. Mineralization can be separated into three distinct stages. Ore fluids are H2O dominant, with approximately 20 to 30 and 10 to 20 mole% CO2 for gold-bearing stage I and II veins, respectively. Vein formation pressures and temperatures were 2.2 kbar and 300 to 370°C for stage I and 1.6 kbar and 250 to 320°C for stage II. The narrow range of δ18O values for ore fluids from the deposits throughout the Xiaoqinling district indicate a common deep fluid source, most likely of magmatic origin. The 34S data suggest sulfur originated from a magmatic fluid of approximately 2 ± 2%, with significant local sulfur contributions leading to large variations in the ore-stage sulfide minerals. The most probable mechanism for the deposition of gold is phase separation caused by the partial loss of volatiles such as CO2 and CH4.

Bolin, Cong; Qingchen, Wang; Mingguo, Zhai

doi: 10.1080/00206819909465172pmid: N/A

Critical but controversial problems in the study of UHP metamorphic rocks from the Dabie-Sulu region include: (1) the possible existence of ophiolitic mélange; (2) the “in situ” versus “foreign” origin of UHP eclogites and their enclosing gneisses; (3) the possible presence and role of fluids during ultrahigh-pressure (UHP) recrystallization; (4) the timing of collision between the Yangtze and Sino-Korean continental blocks; (5) the polarity of syncollisional subduction; and (6) a single-versus multistage exhumation scenario for the UHP rocks. These questions are discussed in light of new geological, geochemical, and isotopic constraints. Our conclusions for the Dabie-Sulu belt are as follows: (1) Mafic-ultramafic blocks are of two distinct origins: one group samples lithosphere of the suprasubductionzone mantle wedge, whereas the second group represents postcollisional magmatic intrusions. Neither lithologic group represents true oceanic crust. (2) Quartzofeldspathic gneisses enveloping the eclogites are of two types— metasedimentary “in situ” and igneous “foreign.” The paragneisses contain UHP garnets + white micas, and are uniformly older (235 ± 5 Ma) than the orthogneisses (210 ± 5 Ma), which are devoid of UHP mineralogic indicators. (3) Fluids were active under UHP conditions and allowed the formation of UHP hydrous phases such as phengite and zoisite. However, the aqueous fluids may have been restricted to certain channels/pathways during exhumation. External fluids were absent until ascent of the UHP rocks to middle-crustal levels. (4) The Yangtze and Sino-Korean continental blocks collided during 230 to 240 Ma, when supracontinental rocks experienced UHP metamorphism. The HP metamorphic event dated as >400 Ma might record a subduction of oceanic crust during the Paleozoic. (5) An ancient mantle wedge is revealed by geochemical characteristics of Mesozoic magmatic rocks developed on the southern margin of the Sino-Korean craton, the hanging wall of the UHP-rock-bearing unit. Seismic tomography images reveal that the Yangtze block extends beneath the Dabie-Sulu orogenic belt. This indicates that both oceanic and continental crust had a northward subduction polarity. (6) Taking petrologic and geochronological data into account, we prefer a multistage exhumation model. The UHP rocks were exhumed rapidly during the first stage (230 to 200 Ma), perhaps reflecting a corner-flow mechanism. Then, buoyancy and mantle upwelling brought the UHP rocks up to middle-crustal levels during the second stage (200 to 170 Ma). Extension and thermal uplift, as well as erosion, eventually exposed the UHP rocks to the surface in the third stage (170 to 120 Ma).

Abbott, Richard N.; Jackson, Trevor A.; Scott, Peter W.

doi: 10.1080/00206819909465173pmid: N/A

Previously undescribed, microscopic textures in partially serpentinized peridotite shed light on the process of serpentinization. Fracturing and fragmentation of the peridotite protolith produced a modest increase in volume and opened channels to reactive aqueous fluids. Prior to, or in conjunction with, the first appearance of serpentine, micrometer-scale grains of magnetite crystallized on the surfaces of fractures. This decoration of fracture surfaces faithfully preserved the original, angular outlines of fragments of forsterite and pyroxene through all stages of serpentinization. Simple geometric considerations indicate that the volume of a given fragment did not change during replacement. Partially serpentinized forsterite shows a penetrative, micrometer-scale, sieve or sponge texture. Fifty percent or more of the volume of optically continuous forsterite can consist of micrometer-scale perforations, filled with serpentine. This sponge texture has not been described elsewhere. The delicate connections between different parts of the same forsterite grain further support strict constant-volume replacement of the fragments. Serpentinization stopped before completion (mid-reaction) when the otherwise open, magnetite-decorated channels themselves filled with serpentine, effectively sealing the rock from further access by aqueous fluids.

Valencia-Moreno, Martin; Ruiz, Joaquin; Roldan-Quintana, Jaime

doi: 10.1080/00206819909465174pmid: N/A

The geochemistry of Laramide granitic rocks from central Sonora was studied to relate possible compositional variations to the assimilation of different crustal sources. Most of the studied rocks are granodiorites collected near the southern boundary of the Paleozoic North American continent. North of this boundary, the Laramide plutons intruded a thick section of Upper Proterozoic and Paleozoic miogeoclinal strata, whereas south of it, the intrusives were emplaced in Lower Ordovician to Permian eugeoclinal rocks accreted to the continental margin between Late Permian and Middle Triassic times. Whole-rock Na2O, TiO2, and P2O5 contents are slightly higher in plutons intruded in cratonic and miogeoclinal rocks, whereas MgO and CaO seem to be higher in plutons emplaced in eugeoclinal rocks. The samples located in the north are characterized by steeper chondrite-normal-ized REE slopes and generally well developed negative europium anomalies, whereas the group of granitoids in the south have flatter REE slopes and generally smaller negative europium anomalies. Available isotope data show 87Sr/86Sr initial ratios above 0.7070, and ϵNd initial values between −4.6 and −4.2 in plutons to the north. In a narrow E-W region just south of the Paleozoic continental margin, the samples yielded similar 87Sr/86Sr ratios but unexpectedly low ϵNd values between −5.4 and −5.1. South of this region, the isotope signatures are slightly more primitive, with 87Sr/86Sr initial ratios between 0.7067 and 0.7057 and initial ϵNd values between −3.9 and −3.7. The results of this study suggest that the nature of the assimilated crust may have influenced the final composition of the Laramide granitic rocks of central Sonora.